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Microwave antenna theory and design silver

MICROWAVE ANTENNA
THEORY AND DESIGN
Ediied by
SAMUEL SILVER
ASSOCIATE PROFESSOR OF ELECTRICAL ENGINEERING
UNNEB.SITY OF CALIFORNIA, i3EP.KELEY
OFFICE OF
SCIENTIFIC RESEARCH AND DEVELOPMENT
NATIONAL DEFENSE RESEARCH COMMITTEE
FIRST EDITION
NEW YORK, TORONTO LONDON
McGRAW-HILL BOOK CO,MPANY, INC.
1949
.,
,.,
MICROWAVE .$xTEN\ $ THEC!R Y .ISD DESIGN
(hPYRIGH,T, 1949, B>- THE
hlC~RA W-HILL
BOOK ~(IIIP.I.NY, lKC.
P31XTEI) lx THE U>-lTEI) STATES OF AMERICA
.111 rights Testwed. This book, or

parts thereof, HI(IY not be reproduced
in any form (rilho?(l prr)rlission of
/he ,L)(//)/ishers,
THE MAPLE PRESS COMPANY, YORK, PA,
*ienCe
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lf[CRO JV.4 VE A NTE.VNA TfZEOR Y
EDITORIAL STAFF
SAMUEL SILVER
HUBERT M. JAMES
AND DESIGN
CO.VTRIB L’TI.VG A PTHORS
J. E. llATON
R. hf. R IZDHEFFER
L. J. I;YGES
J. R.
RISSER
T. J. KEARY
S. SILVER
H. KRUTTER
O. A. TYSON
(2. G. hlAcFARL.4NE
L. C. \’AN ATTA
Foreword
T
HE tremendous research and development effort that ~vent into the
development of radar and related techniques during }Vorld IJ ar II
resulted not only in hundreds of radar sets for military (and some for
possible peacetime) use but also in a great body of information and ncm
techniques in the electronics and high-frequency fields. 13ecause this
basic material may be of great value to science and engineering, it seemed
most important to publish it as soon as security permitted.
The Radiation Laboratory of 311T, ~vhich operated under the super-
vision of the National Defense Research (’ommittec, undertook the great
task of preparing these volumes.


The ~vorl{described berein, ho\\-eyer,is
the collective result of ~vork done at many laboratories, Army, Xavy,
university, and industrial, both in this country and in JZngland, (<anada,
and other Dominions.
The Radiation Laboratory, once its proposals ]vere approved and
finances provided by the Office of Scientific Research and l)evelopment,
chose Louis N. Ridenour as Fklitor-in-(’bief to led and direct tbe entire
-,
project. An editorial staff ]vas then selected of those best quulificd for
this type of task. Finally the authors for the various volumes or chapters
or sections were chosen from among those experts ~vho ~t-ereintimately
familiar with the various fields, and ]vbo \vere able and willing to ]vrite
the summaries of them. This entire staff agreed to remain at ~vork at
MIT for six months or more after the \\-orkof the Radiation I.aboratory
was complete. These volumes stand as a monument to this group.
These volumes serve as a memorial to tbe unnamed hundreds and
thousands of other scientists, engineers, and others ]vho actually carried
on the research, development, and engineering work tbe results of which
are herein described. There ~vereso many involved in this ~vorkand they
worked so closely together even though often in \\-idelyseparated labora-
tories that it is impossible to name or even to know those ]vho contributed
to a particular idea or development,
(My certain ones ~vho~u-ote reports
w- or articles have even been mentioned, But to all those ~vhocontributed
~ in any way to this great cooperative development enterprise, both in this
~ country and in England, these volumes are dedicated,
a
L. A. DLTBRIDGE.
z 1,!
Preface
T
HE need that arose during the ]var for utilizing the microwave region
of the radio frequency spectrum for communications and radar stimu-
lated the development of nelv types of antennas. ‘l’he problems and
design techniques, lying as they do in the domain of both applied electro-
magnetic theory and optics, are quite distinct from those of long-wave
antennas.
It is the aim of the present volume to make available to the
antenna engineer a systematic treatment of the basic principles and the
fundamental microwave antenna types and techniques. The elements
of electromagnetic theory and physical optics that are needed as a basis
for design techniques are developed quite fully. Critical attention is
paid to the assumptions and approximations that are commonly made
in the theoretical developments to emphasize the domain of applicability
of the results. The subject of geometrical optics has been treated only
to the extent necessary to formulate its basic principles and to sho~v its
relation as a short wavelength approximation to the more exact methods
of field theory. The brevity of treatment should not be taken as an
index of the relative importance of geometrical optics to that of electro-
magnetic theory and physical optics.
It is in fact true that the former
is generally the starting point in the design of the optical elements
(reflectors and lenses) of an antenna. However, the use of ray theory
for microwave systems presents no new problems over those encountered
in optics—on which there are a number of excellent treatises—except
that perhaps the law of the optical path appears more prominently in
micro~vave applications.
In the original planning of the book it was the intention of the editors
to integrate all of the major wQrk done in this country and in Great
Britoin and Canada. This proved, however, to be too ambitious an
undertaking. Nfany subjects have regrettably been omitted completely,
and others have had to be treated in a purely cursory manner.
It \vas
unfortunately necessary to omit two chapters on rapid scanning antennas
prepared by Dr. C. V. Robinson.
The time required to revise the
material to conform ~vith the requirements of military security and yet
to represent an adequate exposition of the subject would have unduly
delayed the publication of the hook. Certain sections of Dr. Robinson’s
material have been incorporated into Chaps. 6 and 12.
ix
x
PREFACE
I take pleasure in expressing here my appreciation to Prof. Hubei-t
M. James who, as Technical Editor, shared with me much of the
editorial work and the attendant responsibilities.
The scope of the book,
the order of presentation of the material, and the sectional division within
chapters were arrived at by us jointly in consultation with the authors.
I am personally indebted to Professor .James for his editorial Ivork on
my own chapters.
The responsibility for the final form of the book, the errors of omission
and commission, is mine. A word of explanation to the authors of the
various chapters is in order.
After the close of the Office of I’ublications
and the dispersal of the group, I have on occasions made use of my
editorial prerogative to revise their presentations.
I hope that the results
meet ~vith their approval. The policy of assignment of credit also needs
explanation. The interpretation of both Professor James and myself of
the policy on credit assignment formulated by the Editorial Board for
the Technical Series has been to the effect that no piece of work discussed
in the text would be associated with an individual or individuals.
Radi-
ation Laboratory reports are referred to in the sense that they represent
source material for the chapter rather than individual acknowledgements.
References to unpublished material of the Radiation Laboratory note-
books have been assiduously a~oided, although such material has been
dramm upon extensively by all of us.
In defense of this policy it may be
stated that the ]vorlc at the Radiation Laboratory was truly a cooperative
effort, and in only a few instances would it have been possible to assign
individual credit unequivocally.
The completion of the book was made possible through the efforis of
a number of people; in behalf of the editorial staff and the authors I wish
to acknowledge their assistance and contributions. Mrs. Barbara Vogel
and Mrs. Ellen Fine of the Radiation Laboratory served as technical
assistants; the production of figures and photographs \vas expedited by
hlrs. Frances Bourget and Mrs. nary Sheats. It proved impossible to
finish the ]t-orl<by the closing date of the Office of l’ublications; the h’aval
Research Laboratory accepted the ~vork as one of the projects of the
newly formed Antenna Research Section and contributed generously in
personnel and facilities.
Special thanks are due to A. S. Dunbar,
1, Katz, and Dr. I. Maddaus for their editorial assistance; to Queenie
Parigian and Louise Beltramini for preparation of the manuscript;
and to Betty Hodgkins who prepared almost all of the figures.
The editors are indebted to Dr. G. G. Macfarlane of the Tele-
communications Research Establishment, Great Britainj for his
critical review of several of the theoretical chapters and his contribution
on the theory of slot radiators in Chap. 9. John Powell of the
Radiation Laboratory prepared material on lenses that was used in
Ch:lp. 11. The S:1( iomd Rcsc:wch (’ouncil of Can:&~ :md the llrit isll
(’entnd Radio 13urw~u h~~vc~rwiously granted us permission to
ti~li(.
m:ltcrial from ( ‘unudi:m :md I;ritish reports in accord:mcc ~~ith mlrrrnt
security U3glllotioms. ‘l>hc I?wII Telephone I.abora,twy supplied the
photographs of mct:d lens antennas.
S.4 MUEL khLVlil{.
K:\v\T, lll)sl’.\1i1lI T.WIMWIIY,
‘!f”lslllxlm)x, l). (’.,
:lprd, 1947.
.
Contents
FORE WORD BY L. A. DUBRmGE . . . . . . . . . . . . . . . . . . vii
PRE1744CE. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ix
CHAP.1. SURVEY OF MICROWAVE ANT~~NNADESIGN PROBLEMS
1
1.1. The WavelcngthRegion. . . . . . 1
1.2. .Lntenna Patterns . . . . . . . . . . . . . . . . . . . 2
1.3. Types of }Iicrowave Beams. 6
1.4. lIicrowave Transmission I,ines . 7
1.5. Radiating llernents . . . . . . . . . 8
16. .4 Survey of kllcrowavc }.ntenna Types 9
1.7. Impedance Specifications. 13
1.8. Program of the Present Volume 14
CH.4P. 2. CIRCUIT RJ31JATIOIW, Rf3CIPR0CiTY THW3RF~!>fS. 16
21.
22.
23.
2.4.
2.5.
2.6.
2.7.
2.8.
2.9.
Introduction . . . . . . . . . . . . . . . . . . . 16
The Four-terminal ~etwork. 17
The Rayleigh Reciprocity Theorcnl 19
Th6venin’s Theorem and the Nfzximum-power Theorem 20
The Two-wire Transmission I,ine 21
The Homogeneous Transmission I,ine 23
The LosslessLin e 26
Transformation Charts. 29
The Four-terminal N“etwork l’:quivalent of a Sectio]l of Trans-
missiOn Line . . . . . 36
TRANSMI~EVG ANn RECEIVING ANTENNAS. . 37
2.10.
2.11.
2.12.
2.13.
2.14.
2.15.
2.16.
2.17.
2.18.
The Antema as a Terminating Impedance 37
The Receiving Antenna System 40
The Transmitter and Receiver as a Coupled System 45
Reciprocity between the Transmitting and Itecei\,ing Patterns of
an Antenna . . . . . . . . . . . . . . . . . . . 48
The .kverage Cross Section for a Matched System 50
Dependence of the Cross Section on Antenna Mismatch 51
The Four-terminal Network Representation. 53
l)evelopment of the N’etwork Equations 56
The Reciprocity Relation between the Transfer Impedance
Coetlicient s, . . . . . . . . . . . . . . . . . . 59
X111
xiv CO.YYfil.VTS
CHAP. 3. R.\ III.iTIOS FR031 CUIUWXT I) ISTRII)I”T1OSS. (iI
31. Tllc Field Ilquations. 6]
32. The (;onstit,ltive Paramctrrs;
I.inc:wity mid SIlperpo sition . 6,5
33. Ilou])dary Contlitions. 66
3 ~. The Field ~q~latio]ls for H:mnonic TIInc ])(, p[,lld(>I1[W 68
3.5. I’aynting’s Thmreln . . . . M)
36. The ll-a\,c k;qll:ltions. 71
3.7. Simple l~avc Sollltions. 73
38. General Sollttion of the Field I}q,,~tiol,s in Tcr],,s of tl)r fk),,rtcs,
for a Ti]llc-pcriodir lri(ld. 8(I
3.9, Field ]),,e to Sollrtt,s in an U]IhoIuI(ltYi Ii(,gion 84
3.10. Field in a licgion Rotmdcd hy S(lrfa((,s of I]lfillltcl}- (’OI)(l\l(tIvr
llc,clia . . . . . . . . . . . . . . . . . . . . . 8fi
311. Tl](, Far-zone Fields 87
312. I’olarization.
!)1
3~13. The I;lcctric l)ipolc !)2
314. Tllc IItignetir lh]mlc 95
3.15. The F:lr-zonr Firl(ls of I,i]l(,-rurr(,nt l)istrilllltif!)ls !)(i
316. The “H:df-~1 IV(, l)ilmlt, ”
!)8
317. Sllpcrpm]tion of l~icl{ls !)!1
31S The 1)0111>1(,-(111)01(System
101
319. I{cgldur Space .Irr:iys
104
C][.IP. 4. ll l’;l; FI{O>-TS :~~1) RAYS 107
41. TIIC II\lygrns-Crccn Forn~lll:L for thr Ill((,tr(l]]]:Lg]l(ti[, l’itl(l 107
42, Gcol]lctrical ()~]tirs: l~”avefronts and l{:lys 110
43. C1lr~:itllrr of the Ilmys in an Inllo]]Iogc,l](,[~!ls Ilcdiunl 111
4+. Energy FlOIv in (lcometrical Optics 112
45. (;comctrical optics :is :LZero-~ra~clcngth I,in]it 114
46. The H~lygens-Frrsnrl Principle and Gconlctriral Optics: The Far-
zonc .kpproximation
116
47. The Principle of Stationm-y I’hasc
11{)
48. Ft=rnlat’s I’rincip]e.
122
4.9, The I,a,v of the optiral Path
125
(’]~ \l,, 5, S(’.LTTERISC, .kN-T) DIFFR.ACTIOX. 12!)
51. (;cncral (’onsidcrntions
129
52. Bol]ndary (’o]lditions
130
53. Iieflection hy an Infillite Plane S(lrt’arc; the l’rinril)lv of ItII:igrs 132
APPROXIMATE JIETHODS FOR REFLECTOI+SOF ;lILBITMARYS]J.\PE
54,
55,
56,
57,
58.
59.
510.
511.
512.
The Geometrical-optics IIcthorl
Calculation of the Scattrrrd Firl[l
Superposition of the So~lrcc l~icl(l :IIId tl)(, Sc:\ttercd l~icld.
The Current-distril, ~ltion 31cthod
Calc(llation of the Scattrrrd Field
Application to Point-source md IJirl{J-sollrre l;ecds.
Reaction of a Reflector on a Point-source Feed
The .Aperture-fielci Ifethod
The Fraunhofer Region.
, 137
138
139
143
144
146
149
155
. .
158
,
160
.
co’YTA”l’7’s xv
l) IF1.ll.\(r10N . . . ,,162
S.IS, (i,r,crtLI
(’~)])si,l,,r:,iit)])s mI tht, .ipproxim:ltc Ilt,tllods 162
514. l{ttlll~,tltJJl to :1 S[alilr l)illr:~(tlf)n I’I’o I)lcIN 164
515. l)~~lli]lct’s l’rl]l{ipl~i for tll(’ l<;lc,~tro]ll:tg])f,ti(, Ir](,ld 167
(’ti~l,. 6.
.lI’KI{TURF; ll.LL’llll-ATIOX” AX]) .l>-TI,;X-XA I’ATTERNS
169
61. l’ril]l:~ry and S[,co]ltlxry l’tlttrms 169
62. Tll(, l)iffr:trtion Fieltl 169
(i 3. I’ouri(r Integral li(,l)rc,s(,]ltati{jl~ of the Fraunhofcr lie~ion
174
64, (+CJI(M1 I:caturts of tht, Sccolld:wy l’~ttvm
175
6,5. TIIC l{rctmlg~ll:~r .fporture ]80
6.ti. Tl!o-(lilllt,nsio]lal Prol)lcms
182
67. I’l]:i.w-error kYfects. .
186
08. TIIC (’irc(llm .ipcrturc
192
&9. Th(, Field o]] the Axis in thr Frcsnrl ]tcgion
]96
(’l[,\],, 7. l[I(:ROIV,.fV}; TRAA-S~f ISSIOA” I,IN-ES 200
71
llicro~j :ivc nnd I,ong-~vave Trimsmission I,illm 200
72. l’rop:l~atio]l in ~f”:~vcgllidcs of l“niform (;ross Swtion 201
73. orthogo]lallty Rcl:ltions and Power Flow. 207
74. Transnlissiun I,inr (’onsidcrations in l~:lvrguidrs 209
75. XctJt ork Kqllivalents of Junrtions and ohstaclcs 214
76. 7’/l.l/-modc Trallsmission I,irlcs 216
77. (’ozxis,l I,incs: ?’~.!f-rnodc 217
7.8. (’oaxial I.ines: T.If - and T]i-nlodcs 219
7.!),
(’:Is,.acIc Tmnsformcrs: TJ~.lf-mode 221
71o. I’arallel St~lhs and Series ILeactancm.
223
711. licctang~llar }Vavcguidcs: I’A’- and ?’,lf-modes 226
712. Impcdanrc Transformers for Iiectangular (;uidcs 229
713. Circular ll-aveguide: T~- and TJf-modcw. 233
7.14. Ivindows for LTSCin Circular Guides 235
715. I’arallel-plate i~aveguide. . 235
716.11esignN Totes . . . . . . . . . . . . . 238
CHAF. 8. lfICROWAVE DIPOLIl A3JTE~~AS ANI) F13f?DS 239
81. Characteristics of Antenna Feeds 239
8.2, Coaxial I,ine Terminations: The Skirt Dipole 240
83. Asymmetric Dipole Termination.
242
84. Symmetrically 13nergizcri Dipoles: Slot-fed Systems 245
85. Shape and Size of the Dipole . 248
86. lVaveguidc-line-fcd Dipoles. . 250
87. Directive Dipole Feeds, . . . 250
88. Dipole-disk Fords . . . . . . . . . . . . . . . . . . 251
89, Double-dipole Feces, ., . . . . . . . . . . . . .253
8.10. Lfulti-dipole Systems, . . . 256
CHAP. 9. LINI?AR ARRAY AXTEiYNAS AND FF23DS 257
9.1. (kmcral Considerations. . 257
XVI
CON TEJVTS
PA~EEN THEORY . . . . . . . . . . . . . . . . . . . . . . 256
92. General Array Formula. . . 258
93. The Associated Polynomial 261
9.4. U1liformArrays . . . . . 264
9.5, Broadside 13e:~nls . . . . 267
9.6. Erl(l-tire I~ean]s . . . . . . . . . 274
9.7. 13ca1u Synthesis . . . . . . . . . . . . . 279
RADIATING EI.EMMNTS . . . . . . . . . . . . . . . . . 284
9.8. llipole Radiators . . . . . . . . . . . . . . 284
9.9. Slots in Jvaveguide }Talk. . 286
9.10. Theory of Slot Radiators. . 287
9.11. Slots in Rectangular J$’aveguide; ‘1’~,,-mode 291
912. Experimental Data on Slot l{adi~tors 295
913. Probe-fedS lots . . . . . . . . . . . . .
299
9.14. fVaveguide Radiators 301
9.15. Axially Symmetrical Radiators . 303
9.16. Streamlined Radiators . . . 310
ARRAYS . . . . . . . . . . . . . . . . . . . . 312
9.17. Loaded-line Analysis. . 313
9.18. End-fire .4rray . . . . . . . . . . . . . . . . . . . . . . 316
BROADSIDE AREAYS. . . . . . . . . . . . . . . . . . . . . . . 318
9.19, Suppression of Extraneous Major I,ohcs . 318
920. ResonantArrays . . . . . . . . . . . . . . . 321
921. Beacon Antenna Systems. 327
922. T$onresonant Arrays . . . . . 328
923, Broadband Systems with >Tormal Beams 331
CHAP. 10. WAVEGUIDE AND HORN FE~;~S. . 334
10.1. Radiation from Waveguide of Arhitrmy Cross Scrtion 334
10.2. Radiation from Circular ~~av(’guide 336
103. Radiation from Rectwwlar Guide. 341
10.4. Waveguide Antenna Feeds
347
105. The Double-slot Feed .
348
10.6. Electromagnetic Horns. . . . . . 349
10.7. hrodes in lplane Sectoral Horns 35o
108. Jfodes in If-plane Sectoral Horns
355
109. Vector Diffraction Theory Applied to Srctoral Horns. 357
10.10. Characteristics of Observed Itadiation l’tittmns from Horns of
Rectangular Cross Section 358
10.11. Admittance of Waveguidc and Horns . 366
10.12. Transformation of the L’-plaI~c HorII .idulittan cc fmm the Throat
tothe Uniform Guide . . . . . . . . . . . . . . . . 369
10.13, Admittance Characteristics of H-plane Sectoral Horms
374
1014. CompoundHorns. ., , . . . . . . . . . . . . 376
10.15 .The Box Horn . . . . . . . . 377
1016. Beam Sllaping hy ~lcans of Obstaclrs in HOHI :md \VaveK{lidv
.4pertm’cs. . . . . . . . . . .
380
10.17, Prcssllrizing and hfatch]ng 383
CHAP. 11. DII:I,IcCTILIC i~X1) lIJ:T:lI,-l’I fTI<: I.I<:XSES 388
11.1. Uses of l,[>lls(>sin 31icro~v~vc ,~ntcmms. 388
I)ll:l,l:C,rRICIJEXSF;S. . . . . . . . . . . . . . . . . . . . . . .
112. l’rill{ilJlrso fI>rsign, .
113, Sinlplc I,cllses IVltllo(it Zoning
114. Zoned l)lclrctric I,cnscs . .
11,5. Usc of lIatcri:tls ~v]th Hi~ll l{cfmctivc Indcxrs
11.6. I)lclcctric I,osscs mnd Tolcranccs on Irons I’aramctcrs.
11.7. Itcflections from L)iclcctric Surfaccw
389
389
390
395
398
399
401
hfETAL-I,L~TELEXSES . . . . . . . . . . . . . . . . . . . . . 402
11.8. Parallel-plate I,ensm, . . . . 402
11.!). Other 31eta-lcr,s Structures. 406
11.10. l[cta-plate I.cns Tolrrancrs 407
11.11. Band,vi(lth of l[etal-plztc I,cnscs; Achromatic Doublets 408
11.12. Iteflections from Surfaces of Parallel-plate I,enses 410
CHAP. 12. PENCILBFAhf AND SIhfPLE
FANA”l;D-BEAN1 ANTEKA”AS 413
PENCIL-BEAMANTENNAS. . . . . . . . ., , 413
12.1. Pencil-beam Requirements and Tcchniqlles 413
12.2. Gcomctriral Parameters 415
123. The Surface-current and Aperture-firldI)istrih(ltions. 417
12.4.
The ILadiation Field of the Reflector
420
12.5. The Antenna Gain., . . . . . . . . . . . 423
126. Primary Pattern Designs for hI:mimizing (lain 433
127. Experimental Itcsldts on %condm-y l’attmns 433
128. Impedance Characteristics 439
12.9. The Vertex-plate lf:~tching Trchniquc 443
12.10. Itotation of I’ohu-ization Technique 447
12.11. Structural Design Problems. 448
SIMFLE FANNED-BEAM ANTENNAS. 45o
12.12. Applications of Fanned Beams and Nfcthods of Pmdllrtion 45o
1213. Symmetrically Cut Parahololds 451
12.14. Feed Offset and Contour Cutting of Reflectors 453
1215. The Parabolic Cylinder and Line Source 457
12.16. Parallel-plate Systems 459
12.17. Pdlbox Design Problems 460
CHAP. 13. SHAPED-BEAM ANTENNAS. .
131.
132.
133.
134.
135.
136.
137.
138.
13.9.
Shaped-beam Applications and Requirements
Effect of a Directional Target Response
Survey of Beam-shaping Techniques.
Design of Extended Feeds. . . .
Cylindrical Reflector Antennas .
Reflector Design on the Basis of Ray Theory .
Radiation Pattern Analysis.
Double Curvature Reflector Antennas
Variable Beam Shape.
465
465
468
471
487
494
497
500
502
508
,
XV1ll
COA’7’1{.V 7’s
CHAP. 14. ANTk;XNA IXSTALLAT1ON I’ROBI,E31S 510
GE~~It.4L SIKVl~Y OF I~Scr.4LLA’rIO~ I’~OBLEMS. 510
14.1. (:rolm(l .lllt(,llnas 510
14.2.SllilJ.illterlI]as,,, . . . . . . . . 511
143, ~lir(,r:ift<intellnzs
512
14,4.
Scanning Antennas on Aircraft 513
14.5. Beacon Antennas on Aircraft 521
522
523
528
537
540
543
543
544
544
545
547
550
552
556
557
557
,.561
564
570
572
573
574
,574
578
580
580
581
,582
,58!5
58A
587
593
593
5{)4
601
604
60!)
ISl)lI;X . . . . . . . . . . . . . 6I.5
.
C’11.IPTI;R 1
SURVEY OF MICROWAVE ANTENNA DESIGN PROBLEMS
]]Y s. SII,V1;ll
1.1. The Wavelength Region.—’I’he designation of the boundaries of
the micro!mve region of tIw rlcct romagmctic spectrum is pllrcly arbitrary,
Tile Iong-]vavelcngth limit IIas Ixxm set v:~riously at 25 (Jr 40 cm, even
at 100 cm. From the point, of vie\vof antenna theory and design techn-
iques, the 25-cm val~le is the most appropriate choice, The short-
wa~’elength limit to )ihich it is possible to extend the present terhniq(les
ll:~snot ~etl)ec>r~rcaclle(i; it isinthcnciglll)or}loo(lof lmm. Accordingly
\veshall cunsi(lcr the microlvave region to extend in wavelength from 0.1
to 25 cm, in frcqllcncy from 3 X 105to 1200 31c/see,
This is the transition region bet\\-eenthe or(linary radio region, in
}vhich the \\-avelengtllis very k~rgecomparwl with the dimensions of all
the components of the system (cxccpt perhops for theh~rge and cumber-
some antennas), and the optical region, in ]t-hich the \vavclengths arc
excessil-ely small. I.ong-\vavc concepts rm(l techniques continue to be
useful in the micro \vave region, and at the same time certain devices
used in the optical regionsllr haslense sandn~irror sarcemployeci.
From
the point of vie}v of the antenna designer the most important character-
istic of this fre(~ucncy region is that the wa~~elengthsare of the order of
magnitude of the dimcmsilmsof conventional and easily handled mechan-
ical devices. This leads to radical modification of earlier antenna
techniques and to the appearance of nefv and striking possibilities,
especially in the construction and use of complex antenna structures.
It follows from elementary diffraction theory that if D is the maximum
dimension of an antenna in a given plane and k the ivavrlength of the
radiation, then the minimum angle }vithin which the radiation can be
concentrated in that plane is
(1)
With microwaves one can thus produce highly directive antennas such
as have no parallel in long-wave practice; if agivendirectivity is desired,
it can be obtained \vith a microwave antenna ]vhich is smaller than the
equivalent long-!vave antenna.
The ease with which these small antennas
can be installed and manipulated inarestricted space contributes greatly
to the potential uses of microwaves.
In addition, the convenient size of
1
2 SURVEY OF MICROIV. t J’E ANTEXAVA DESIG.V PROBLEMS [SEC,1.2
microwave antenna elements and of the complete antenna structure makes
it feasible to construct and use antennas of elaborate structure for special
purposes; in particular, it is possible to introduce mechanical motions of
parts of the antenna with respect to other parts, with consequent rapid
motion of the antenna beam.
The microwave region is a transition region also as regards theoretical
methods. The techniques required range from lumped-constant circuit
theory, on the low-frequency side, through transmission-line theory, field
theory, and diffraction theory to geometrical optics, on the high-fre-
quency side. There is frequent need for using several of these theories
in parallel—combining field theory and transmission-line theory, sup-
plementing geometrical optics by diffraction theory, and so on. Optical
problems in the microwave antenna field are relatively complex, and
some are of quite novel character: For instance, the optics of a curved
two-dimensional domain finds practical application in the design of
rapid-scanning antennas.
1.2. Antenna Patterns Before undertaking a survey of the more
important types of microwave antenna,
it will be necessary to state
precisely the terms in which the performance of an antenna will be
described.
The Antenna as a Radiating Device:
The Gain Function.—The field
set up by any radiating system can be dirided into two components:
the induction field and the radiation field. The induction field is impor-
tant only in the immediate vicinity of the radiating system; the energy
associated with it pulsates back and forth between the radiator and
near-by space. At large distances the radiation field is dominant; it
represents a continual flow of energy directly outward from the radiator,
with a density that varies inversely with the sq~iarc of the distance and,
in general, depends on the direction from the source.
In evaluating the performance of an antenna as a radiating system
one considers only the field at a large distance, where the induction field
can be neglected. The antenna is then treated as an effective point
source, radiating power that, per unit solid angle, is a function of direc-
tion only. The directive properties of an antenna are most con~eniently
expressed in terms of the “gain function” G(6’,O). I/et 6’and @ be respec-
tively the colatitude and azimuth angles in a set of polar coordinates
centered at the antenna. Let F’(O,@) be the power radiated per unit
solid angle in direction 0, @ and P~ the total power radiated.
The gain
function is defined as the ratio of the power radiated in a given direction
per unit solid angle to the average power radia~ed per unit solid angle:
47r
(2)
SEC. 1.2]
ANTENNA PATTERNS
3
Thus G(L9,~) expresses the increase in power radiated in a given direction
by the antenna over that from an isotropic radiator emitting the same
total power; it is independent of the actual power level. The gain
function is conveniently visualized as the surface
r = G(f3,@) (3)
distant from origin in each direction by an amount equal to the gain
function for that direction. Typical gain-function surfaces for micro-
wave antennas are illustrated in Fig. 1.1.
The maximum value of the gain function is called the “ gain”; it
will be denoted by GM. The gain of an antenna is the greatest factor
by which the power transmitted in a given direction can be increased
by using that antenna instead of an isotropic radiator.
The “transmitting pattern” of an antenna is the surface
(4)
it is thus the gain-function surface normalized to unit maximum radius.
A cross section of this surface in any plane that includes the origin is
called the “polar diagram” of the antenna in this plane. The polar
diagram is sometimes renormalized to unit maximum radius.
W-hen the pattern of an antenna has a single principal lobe, this is
usually referred to as the “antenna beam. ” This beam may have a
wide variety of forms, as is shown in Fig. 1.1.
The Antenna as a Receiving Dwice: The Receiving Cross Section .—The
performance of an antenna as a receiving device can be described in
terms of a receiving cross section or receiring pattern.
A receiving antenna will pick up energy from an incident plane wave
and will feed it into a transmission line which terminates in an absorbing
load, the detector. The amount of energy absorbed in the load will
depend on the orientation of the antenna, the polarization of the wave,
and the impedance match in the receiving system.
In specifying the
performance of the antenna, we shall suppose that the polarization of
the wave and the impedance characteristics of the detector are such that
maximum power is absorbed. The absorbed power can then be expressed
as the power incident on an effecti~-c absorbing area, called the “ receiving
cross section, ”
or “absorption cross section” A, of the antenna.
If S is
the power flux density in the incident wave, the absorbed power is
P, = ASA,
(5)
The receiving cross section will depend on the direction in which the
plane wave is incident on the antenna.
We shall write it as A, = A,(d,I$),
where o and @ are the spherical angles, already defined, of the direction
4 SL’RJ’E1’ OF JIIC’lK)IV.4 J’E .4.V7’E.\.VA DI<,SIG.I 1’1{01$1.1<.11.V [SW. 12
of incidence of the lvave, This function, like the gain function, is repre-
sented conveniently as the surface
?’ = .4, (0,0).
(6 J
The “ receiving pattern”
of an antenna is drfincd, :malogolls]y t(
the transmitting pattern, as the above surface normalized to unit maxi-
mum radius:
(7)
It is a consequence of the reciprocity theorem to be discllssed in
Chap. 2 that the receiving and transmitting patterns of an antenna are
identical:
~(~,o) = ~r(g:y),
G.,,
A,.,,
(8)
It will also be shown that the ratio .4, u‘0 v is a constant for all matched
antennas:
lr,f ~ ~,
“G
41r
Thus for any matched receiving system
,
A,((l, @l) = :; G(e,l+).
(9)
(lo)
Coverage Pattern, One Way The characteristics of an antenna may
also be described in terms of the performance of a radio or radar system
of which it is a part. It is necessary to distinguish between the case of
one-way transmission, in which a given antenna serves for transmission
or for reception only, and the case of radar or two-way transmission, in
which a single antenna performs both functions.
We consider first a transmitting antenna and a receiving antenna
separated by a large distance R.
Let G, and G, be the respective gain
functions of the two antennas for the direction of transmission. If the
total power transmitted is P, the power radiated in the direction of the
receiver, per unit solid angle, will be (1/4m)PG~. The receiving antenna
will present a receiving cross section (1/’47r)G,x2to the incident wave; it
will, in effect, subtend a solid angle G,A2/’47rRzat the transmitter.
The
power absorbed at the receiver will thus be
(11)
The maximum operating range is determined by the signal-to-noise
ratio of the detector system. If P,m is the minimum detectable signal
for the receiver, the maximum operating range is
R
(-)
P$i A
.,., =
P
~; (G,G,)’ ~
,m
(12:
SEC.12] ANTENNA PATTERNS
5
Thus, if it is possible to ignore the effect of the earth on the propagation
of the wave and if G, is constant, it will be possible to operate the receiving
system satisfactorily everywhere within the surface
(13)
where the transmitter is taken to be at the origin.
This surface will be
called the “free-space coverage pattern for one-way transmission. ”
Coverage Pattern, Two Ways. - -In most radar applications the same
antenna is used for transmission and reception.
One is here interested
in detecting a target, which may be characterized by its ‘(scattering
cross section” u. This is the actual cross section of a sphere that in the
same position as the target would scatter back to the receiver the same
amount of energy as is returned by the target. For this fictitious iso-
tropic scatterer, the effective angle subtended at the transmitter is U/R2
and the total power intercepted is
(14)
Scattered isotropically, this power would appear back at the transmitter
as a power flux, per unit area,
(15)
Actually, the scattering of most targets is not uniform. The scattering
cross section of the target will in any case-be defined by Eq. (15), but it
will usually be a function of the orientation of the target.
The power absorbed b:- the receiver from the scattered wave will be
P,= A+S=R
(16)
since here G, = G,. If the effect of the earth cm transmission of the
waves can be neglected, it will be possible to detect the target only when
it lies within the surface
(17)
about the transmitter as an origin.
This surface will be called the “free-
space coverage pattern for twe-way transmissi,m. ”
The extent of the coverage patterns is determined by characteristics
of the system and target—output power, receiver sensitivity, target size
—that are not under the control of the antenna designer. The form of
the coverage patterns is determined by but is not the same as the form
of the antenna transmitting a,nd receiving patterns; in the coverage
patterns, r is proportional to [G,(o,r#J)]Jfirather than to G,(o, +). The
6 SURVEY OF MICROWA FE AIV7’EiVA’44 DESIG.V PROBLEWS [SEC.13
desired form of the coverage pattern is largely determined by the use to
be made of the system. From it, one can derive the required form of the
transmitting or receiving pattern of the antenna; it is usually in terms of
this type of pattern that antenna performance is measured and specified.
It is to be emphasized that the discussion of coverage patterns gi~en
(b)
(c)
(d)
FIG.I.I.—Typicalgain-functionsurfacesformicrowaveantennas. (a)Toroidal(omni-
directional)pattern;(b)pencil-beampattern;(c) flat-topflaredbeam;(d)asymmetrically
flaredbeam.
here assumes free-space conditions. In many important applications,
coverage is affected by interference and diffraction phenomena due to
the earth, by meteorological conditions, and by other factors. A detailed
account of these factors, which may be of considerable importance in
determining the antenna transmitting pattern required t“ora given appli-
cation, will be found in Vol. 13 of the Radiation I,aboratory Series.
103. Types of Microwave Beams.—The most important types of
microwave beams are illustrated in Fig. 1.1.
The least directive beam is the “toroidal beam,” 1which is uniform in
1Such a beam is also referredto as “omnidirectional.”
(IRE Standardsand
Definitions,1946.)
SEC.1.4]
MICRO WAVE TRANSMISSION LINES
7
azimuth but directive in elevation.
Such a beam is desirable as a marker
for an airfield because it can be detected from all directions.
The most directive type of antenna gives a “pencil beam, ” in which
the major portion of the energy is confined to a small cone of nearly
circular cross section.
With the high directivity of this beam goes a
very high gain, often as great as 1000. In radar applications such a
beam may be used like a searchlight beam in determining the angular
position of a target.
Although the pencil beam is useful for precise determination of radar
target positions, it is difficult to use in locating random targets. For
the latter purpose it is better to use a “fanned beam,” which extends
through a greater angle in one plane than it does in a plane perpendicular
to that plane. The greater part of the energy is then directed into a cone
of roughly elliptical cross section, with the long axis, for example, ver-
tical. By sweeping this beam in azimuth, one can scan the sky more
rapidly than with a pencil beam, decreasing the time during which a
target may go undetected. Such a fanned beam still permits precise
location of targets in azimuth, at the expense of loss of information
concerning target elevation.
Other applications of microwave beams require the use of beams with
carefully shaped polar diagrams.
These include one-sided flares, such
as is illustrated in Fig. 1Id, in which the polar diagram in the flare
plane is roughly an obtuse triangle, whereas in transverse planes the beam
remains narrow. In radar use, such a beam at the same time permits
precise location of targets in azimuth and assures most effective distribu-
tion of radiation within the vertical plane of the beam. Toroidal beams
with a one-sided flare in elevation have also been developed.
No theoretical factors limit any of the above beam types to the micro-
wave region, but many practical limitations are imposed on long-wave
antennas by the necessary relationship between the dimensions of the
antenna elements and the wavelengths.
104. Microwave Transmission Lines The form of microwave
antennas depends upon the nature of the available radiating elements,
and this in turn depends upon the nature of the transmission lines that
feed energy to these elements. We therefore preface a survey of the
main types of microwave antennas with a brief description of microwave
transmission lines; a detailed discussion of these lines will be found in
Chap. 7.
Unshielded parallel-wire transmission lines are not suitable for micro-
wave use; if they are not to radiate excessive y, the spacing of the wires
must be so small that the power-carrying capacity of the line is severely
limited.
Use of the self-shielding coaxial line is possible in the microwa~ t~
8
S’( “I<i’l{~- 01” .ifl(:l{() WA }’1< .1i< 7’liA’.YA DKSIG.V PII’OI{LE.IIS [SE<. 1,5
region but is generally restricted to lfa~-elengths of approximately 10 cm
or more. IJor proper action as a transmission line, a coaxial line slLoulcf
7
(b)
(c)
transmit electromagnetic Ir:lves in only
a single mode; other\\ise the generator
100!{s into an indetermirmte impedance
and tends to be erratic in operation.
on this account it is necessary to keep
the at-erage circumference of inner and
outer condllctors less than the frce-
space wavelength of the transmitted
~ravcs, .\t ~vavelengths shorter than
10 cm this limitation on the dimensions
of c~mxial lines begins to limit their
polvcr-carrying capacity to a (Ir=gree
that m~kes them lmsatisfactory for
most purposes.
The most ~lseful transmission line
in the miclotvave region is the hollo\\-
pipe. Sllrll pipes \vill sllpport the
propagatiorr of :lrleiect,rom:~g~lrti(,!j-:~~e
only it-hen they are sufficiently large
comp:we(l \\ith its free-space \vave-
length. As g~lides for long-lrave
radi:Ltl(Jn, ]nt,oleral)ly large pipes are
reql[ir(,(l, I)llt in the microlrave region
it lxw)mes pf)ssit)le to msepipesof rmn-
vcnlcnt, SIZC.
I,ike the coaxi:d gllide,
there is :Llsfjan llpper limit imp(w(lon
the crow-sectional dimension of the pipe
if it, is to tr:msmit the \v:ive in only N
single mo(le.
II()\\-e\-er,in theal)scnce
of :ln inner con(lllctor, this size limit:L-
tion (l(wnot :Ifl’ect the Ix)li-cr r:llxwily
so seril}llsly :Wit does in tllc c(mi:~l line.
1.5. Radiating Elements.—T he
natllre
of thc ra(li:~ting elements
trrmin:~ting :L transmission line is to
:L (l) USi(l(>I’:Ll)l(’ (’Xt C’llt (1(’t(’l’lllill(’(1 })~
tile
n:~tllre of the li]le itwlt’. ‘1’y])ieal
l~,ng-lv:~le r:l(li:~tillg clenwnt + :Irr the
“{lil)ole’”
:lrl((~]l]l:ls,sll~,ll:1stllc (,t~lltcr-
(Iril.en i]:llf-\\:l\-e (Iil)tlle,
nll(l loop
coaxial lines lend themselves to sllch terminations.
Many long-wave
antenna ideas have lwen rarr-ied uver into the micro\rave region, par-
tic~~hwlythose connected with thehalf-]rave dipde; the tramsitiorr, ho\v-
ever, is riot rnereiy a mattrr of wovelcmgth scaling.
In a microl}ave
antenna tl~e cross-sertional dimensions of the transmission line are com-
partihlc to the dimensions of the half-~vavc dipole, and consequently, the
coupling lmtween the radiator and tile line becomes a more significant
prol)lem tlian in a corresp(jnclin~ Iong-ivave system. The cross-sectional
dimensions of the dipole element are dso comparable to its length. A
typi~al microwave dipole is shown in Fig. 1“2c; the analysis and undt=r-
stancling of S1lC}Imicro}vave dipoles is at best still in a qualitative stage.
The ose of hollow ~vaveyuide lines leads to the employment of entirely
(Lffc,rent radiating systems.
The simplest radiating termination for such
a line is j~lst the open end of the g~lirle, through which the energy passes
into space. The dimensions of the mouth aperture are then comparable
to the wavelength; as a result of diffraction, the energy does not continue
in a lwam corresponding to the cross section of the pipe but spreads out
considerably about, the direction of propagation defined by the guide.
The degree of spreading depends on the ratio of aperture dimensions to
wa~’ekmgth. On flaring or constricting the terminal region of the guide
in order to control the directivity of the radiated energy, one arrives at
electromagnetic horns based on the same fundamental principles as
acoustic horns (Fig. 1.20!).
Another type of element that appears in microwave antennas is the
radiating slot (Fig. 1.2r). There is a distribution of current over the
inside wall of a waveguide associated with the wave that is propagated
in the interior. If a slot is milled in the wall of the guide so as to cut
across the lines of current flow, the interior of the guide is coupled to
space and energy is radiated through the slot. (If the slot is milled along
the line of current flow, the space coupling and radiation are negligible. )
I slot will radiate most effectively if it is resonant at the frequency in
question. The long dimension of a resonant slot is nearly a half \\-ave-
iength, and the transverse dimension a small fraction of this; the perim-
eter rJt”the slot is thus closely a wavelength.
1.6. A Survey of Microwave Antenna Types.—We are now in a posi-
tion to mention briefly the principal types of antennas to be considered
in this book.
Antennas jo~ Toroidal Beams.—A toroidal beam may be produced
by an isolated half-wave antenna.
This is a useful antenna over a large
frequency range, the iimit being set by the mechanical problems of sup-
porting the antenna and achieving the required isolation. The beam
thus produced, however, is too broad in elevation for many purposes.
A simple system that maintains azimuthal symmetry but permits
control of directivity in elevation is the biconical horn, illustrated in
10 ASURVEY OF MIC’ROW.41’E .4,1’7’fl,V.VAI)EL7[G.N-PRO13LJY.tf,9 [Sm. 16
Fig, 13. The primary driving element between the apexes of the coues
is a stub fed from a coaxial line. The spread of the energy is determined
by the flare angle and the ratio of mouth dimension to wavelength.
Although this antenna is useful ov~r a
large freq~lency range, maximum di-
rectivity for given antenna ~veightand
size is obtairmble in the microwave
region, where the largest ratio of
aperture to wavelength can be
realized.
Increased directivity in a toroidal
beam can also be obtained with an
array of radiating elements such as
dipoles, dots, or bimnical horns built
up along the symmetry axis of the
beam. The directivity of the array is
determined by its length measured in
~vavelengtbs; high directivities
arc
conveniently obtained by this method only in the microlvave region. .1
typical microwave array of this type is shoum in Fig. 1.4.
Pt,ncil-brum A nfrnnas Bearr~s
thathare direr tivit y both in eleva-
tion and azimuth may be pr(xlllccd by a pair of dipole elements or by a
dipole with a reflecting plate.
The major portion of the energy is con-
tained in a cone ~rith apex angle somewhat less than 180”.
FIG. 14 -–.4. mirmwa~c lmaron array.
Similar beams arc prodllced by horn antennas that permit control
of the directivity throllgh choice of the flare tingle and the n~{)lltl]dimen-
sions. Horns are useful at lo\ver frequencies as JVC1las in the rnicrolrave
region; indeed, the early work on horns Ivas done for \~ti\-elengthsranging
from 50 to 100 cm.
More directive healns-trlic pencil bemns-can be prtd~lced b,v
building up space aryays of the almw systems. T\\-,)-tiimensiorlalarrays
(mattress arrays) an{i mldt,i,mit horn systems arc IISC(Iat l,,,-er frequen-
cies. Their dircctivity is severely limited, ho\\-ever,hy tl~e ]nrtll:mical
problems occasioned by the rcc(llired ratio of (Iimrnsions to }f
:L,t,-
Iengths. Such arrays have not been employe(l in tlie micro~~-averegif)n.
$.Ec. 1.6]
A SURVEY OF MICROWAVE ANTENNA TYPES
11
At these wavelengths it becomes feasible, and indeed very convenient,
to replace the two-dimensional array technique by the use of reflectors
and lenses.
(a)
(b)
FIG. 1.5.—Pencil-beam antennas.
(a) ParaboloidaI mirror; (b) metal-plate lens. (Metcd-
plate lens photo~aph courtes~ of the Bell Telephone Labordorie8.)
Highly directive pencil beams are produced by placing a partially
directive system such as the double-dipole unit, dipole-reflector unit, or
horn at the focus of a paraboloidal reflector or x ccntrosymrnetric lens,.
The use t)f these devices is based
(JIIthe r{)ncrI>ts of ray optics, a(cor(linx
to lvhich thr reflector or Ims takes the dilcrgrt]l ra~s fr<)m tlm pt~int
source at the ftwl[s and converts tlwrn into :L beam of par:dlcl rays.
Despite the diffraction r[fects which limit thr npplicati{)rr of ray optics
and are very important in the micro \~ave regi(m, it is pr:w(i{,nllle to
make the apert[~rm so large that extremely sh:lrp lxmms can lx’ pr(xlllcrd.
Conversely, it is possihlc to ol~tain gwd (lirecti~-it.v Ii-ith :m antcnn:l s()
snmll that :~ircrof’t installations are prLwtiral.
Paralj(jl(Jill:Gl:m(l Imra-
bolic reflectors arc Imrd at lower frrilllencies in s,,rne spcri:ll r:Lsrs, }),lf
in the rcqllircd Iargc sizes they tend to be less
S:LtlSfU; t[)I’~tll:Ln nl:Lttrrss
arrays,
Plastic lenses arc used in the mirro\va~c rrgion in prrcisely the same
\vay as g]:Lss lrnses in the optical region.
In ad(lition, a neir device,
the metal lens, has been de~-elopxl for micro!vaves. ‘L’he fwl~-elcngtl~
of an clertromagnctic ~va~-ein an air-tilled )~avcg~lide is grratrr than that,
in free space; from the optical point of vic)v the ]raveg~lide is a region
of index of rcfractiorr less t}lan unity.
A stack of ]!-aregtlidcs thins c~,n-
stitutes a refractive medium analogous to dielectric material, from lvhich
a metal lens can be fashioned, Figlu-e 1.5 shoivs micro~wi~~eprmcil-
}]eam an{ cnnas employing, respectively, a paralmloidal mirror and a
metal lens as directive devicm.
.4ntennas ,for Flared i3cams.—Simple flared beams and one-sided
flares arc Iikc\vise prod~lced by means of reflectors and lenses and by
arrays of dipole-reflector units or radiating slots.
S\lch arrays by t}lem-
selves give beams that are highly directive in planes containing the array
axis but are fairly broad in the transverse plane.
In order to gain greater
directivity in the transverse plane the array may be used as a line so~u-ce
along the focal line of a parabolic cylindrical reflector; this focuses radia-
tion from a line source in the same \vay that a reflector in the form of a
paraboloid of revol~ltion focuses radiation from a point source. By
suitable shaping of the cross section of the cylinder, one can produce
beams with carefully controlled one-sided flares and other useful special
characteristics. Typical rnicrotrave antennas of this type are sholvn in
Fig. 1+.
Except for a few types of linear array, all micro]vave antennas use
primary sources of radiation together \vith reflectors and lenses. The
radiating element, wiich extracts po\ver directly from the transmission
line, is spoken of as the “ primary feed, ” the “ antenna feed, ” or simply
the “feed”; its radiation pattern as an isol~ted unit is kno]vn as the
“primary pattern” of the antenna.
In combination with the optical
elements of the ~ntenna, the feccf produces the o~-er-all pattern , { thn
antenna, often referred to as the “ secondary pattern” of the antenna.

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